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Aristotelian Essentialism: Essence in the Age of Evolution

Christopher J. Austin

Abstract

The advent of contemporary evolutionary theory ushered in the eventual decline of Aristotelian Essentialism (?) ? for it is widely assumed that essence does not, and cannot have any proper place in the age of evolution. This paper argues that this assumption is a mistake: if ? can be suitably evolved, it need not face extinction. In it, I claim that if that theorys fundamental ontology consists of dispositional properties, and if its characteristic metaphysical machinery is interpreted within the framework of contemporary evolutionary developmental biology, an evolved essentialism is available. The reformulated theory of ? offered in this paper not only fails to fall prey to the typical collection of criticisms, but is also independently both theoretically and empirically plausible. The paper contends that, properly understood, essence belongs in the age of evolution.

Within contemporary philosophy of biology, there is perhaps no greater maligned theory than Aristotelian Essentialism (?). Now that the rosy dawn of Aristotelian metaphysics has faded into twilight1, citing the essence of an organism as an explanatory principle is indicative either of a rather hopeless scientific naivet? or else a dogmatic entrenchment in scholasticism. It is generally agreed that the sun set upon ? for a simple, yet powerful reason: the advent of evolutionary theory. According to the implications of that theory, kind-essences are an ontological superfluity which the world not only has no need of, but simply cannot countenance. However, evolutionary theory has recently had its own paradigm shift, ushered in with the rise of the union between it and developmental theory. With its increasing emphasis on modular, structural explanations of morphological novelty and variation, evolutionary developmental biology (evodevo) has arguably prompted a substantial reshaping of our understanding of the very nature of biological individuals. In light of this reformation, the question naturally arises: what is it to be the what-it-is-to-be of an organism? In what follows, I suggest that the answer to that question is one best interpreted within the ontological framework of ?. I contend that, properly understood, essence belongs in the age of evolution.

Aristotelian Essentialism vs. Evolution First things first: what exactly is ?? One can find many distinct (though often overlapping) definitions in the literature, but here, for the sake of simplicity, and without wishing to rehearse decades of debate, I focus on a simple three-point definition. An Aristotelian essence is (a) comprised of a natural set of intrinsic properties which (b) constitute generative mechanisms for particularised morphological development which (c) are shared among groups of organisms, delineating them as members of the same ,,kind. Regarding (a), the set of properties that comprise an essence and define a natural kind cannot be extrinsic, or relational properties ? abstract properties of phylogenetic lineage or interbreeding relations, etc. ? and their being a collection must not be a result of our conceptual practices (on account of discipline convention or theoretical interests), but instead reflect a structural grouping that is mind-independent: the set of properties that define a natural kind are chosen by nature, not nous. Regarding (b), the properties that comprise an essence are causal properties, teleologically "directed toward" particular anatomical and eidonomical ends.

1 I borrow this colourful phrasing from Hacking (2007).

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Thus ? presents, as Lennox (1987: 340) puts it, a type of ,,teleological essentialism: these groups of properties are causally responsible for the continual shaping and organising of the particularised ontogenic development of the organisms which possess them.2 Regarding (c), being possessed by a great number of organisms and reflecting the ways in which nature is "carved at the joints", these property groupings sort organisms into kinds, functioning as inductively rich ,,information stores about their members typical morphological development.3 That said, although the essences of ? are certainly typological ? in that they sort organisms into developmental types - they are not taxonomical: strictly speaking, ? is a thesis about the source and nature of ontogenic development, not a methodological prescription for classificatory definitions.4

Importantly, note that in endorsing (a)-(c), ? entails a rather particular ontological commitment concerning fundamentality. For the defender of ?, what is fundamental about the world (and is thus primitively explanatory with respect to the biological realm) is stability, or invariance ? it is the unchanging, shared set of kind-defining intrinsic properties which ontologically "lie at the bottom" of organisms. Variation on the other hand, according to ?, is therefore non-fundamental: the wide range of property mosaics peppered throughout the organisms that populate the natural world are an ontological result of, and hence must be explained by reference to, the invariant natures ? or essences - of those organisms. It is the endorsement of this ontological priority, and its declaration of the dominance of the unchanging over the changing, which has engendered what has long been viewed as the most singular error of any essentialist theory in biology: for ?s commitment to the so-called ,,natural state model is nearly universally understood as standing in direct conflict to the contemporary cornerstone of our understanding of the biological realm ? namely, the theory of evolution and the process of natural selection. Indeed, ? has been thought by many to be "...precisely the ,,typological perspective...that Darwin had to displace".5

Because I take it that ?s commitment to the natural state model is the primary source of the contemporary distaste for that theory, I will consider it in detail here. When the ,,natural state model is mentioned, it is likely that most will have in mind Mayrs (1976: 27) popular (and overtly Platonic) phrasing that, on this model, "...[t]here are a limited number of fixed, unchangeable ,,ideas underlying the observed variability [in nature], with the eidos (idea) being the only thing that is fixed and real, while the observed variability has no more reality than the shadows of an object on a cave wall"; here the essences of natural kinds are the Platonic eidoi, while the various distinct instances of the morphogenetic profiles of those kinds are the observed variability. This statement is rather incendiary but, as gestured at in the outline above, the general idea is correct, and ? is committed to it: differentiation of specific property exemplification among members of the same natural kind comes about via various "accidental" exploitations of its essential properties, these properties themselves being both (kind-) defining and unchanging. But of course, by the lights of evolutionary theory, this view of the world simply has it backwards: it is variation which is primitive, not fixity. As Gould (1985:160-161) puts it, "[v]ariation is the raw material of evolutionary change [which] represents the fundamental reality of nature, not an accident about a created norm".6

This view is bolstered by the fact that, just as one would expect if the stasis that the natural state model posits were not fundamental, the vast majority of empirical evidence suggests that there are no ontologically privileged sets of properties - genetic or otherwise - which all members of (purported)

2 Cf. Wilkins (2013), Devitt (2008). 3 As Elder (2008: 345) notes: "If a plurality of organisms is to populate a genuine natural kind...more is needed than just that the same phenotypic traits crop up in member after member of the plurality. The same traits must recur across all the organisms for a common reason". 4 Cf. Lennox (2001), Pellegrin (1987), and Balme (1987). 5 Griffiths (2002: 77). 6 Cf. Okasha (2002: 191).

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natural kinds share.7 The lack of such evidence, as Okasha (2002: 196) rightly points out, has led "virtually all philosophers of biology [to] agree that...it is simply not true that the groups of organisms that working biologists treat as [members of the same natural kind] share a set of common morphological, physiological or genetic traits which set them off from other [kinds]".8 And whats perhaps worse, not only is it nearly universally agreed upon that the invariability posited by the natural state model is nowhere to be found, it is likewise agreed that such invariability runs counter to the very core of the evolutionary world-view: for the sine quo non of the process of natural selection is the existence of a substantial amount of phenotypic and genetic heterogeneity among (purported) members of the same natural kind.9

Furthermore, as Sober (1980: 374-377) convincingly argues, even if there were a stable and unchanging set of properties shared among members of a delineated natural kind, those properties would be incapable of playing the role that ? requires of them. According to (b), the essence of a natural kind is an intrinsic set of causal, goal-directed properties which define a particular developmental path towards a specific ,,morphological profile (according to its kind). In the parlance of the natural state model, the essence of a natural kind is causally productive of an intrinsically privileged developmental plan, one which represents the ,,natural state of the members of that kind, with variations on this natural state representing destructive deviations attributable to the pervading un-natural causal influence of an organisms environment. However, while the discovery of the phenomenon of phenotypic plasticity has taught us that a developing organisms environment is an important and major source of phenotypic variation, it has also taught us that there simply is no such thing as an environment-independent phenotypic trait ? and hence, by extension, no such thing as a natural state, produced purely by an intrinsically specified pathway of particular morphological development.10 Indeed, more and more empirical research suggests that the developmental specification of morphological features via environmental stimuli is not only a functionally ubiquitous phenomenon, but one that may play a vitally important part in the evolutionary process.11 Thus, even if we were to grant that there exist shared sets of intrinsic properties among groups of organisms that demarcated them as members of a particular natural kind, ?s requirement that this intrinsic essence must function ? and do so in some way independently of the extrinsic environment ? as the "prime mover" with respect to an organisms specified morphological development is a theoretical demand which the biological realm does not and cannot meet.

The Evolution of Essentialism These critiques of the natural state model collectively function as a powerful reason for abandoning the metaphysical machinery of ? ? this I do not wish to deny. What I deny, as I will argue below, is that these critiques must sound the theorys death knell. To my mind, the lesson they teach is not that ? ought to be extinct, but rather that if the theory is to survive in a contemporary landscape it, like all else, must evolve. Thus Ive no hesitancy in affirming that the objections of the previous section are devastating for a certain na?ve, primitive form of ? ? perhaps even for the form which Aristotle himself advanced. Indeed, I am more than happy to let the specific letter of that primitive progenitor pass away, if there is available a novel, more sophisticated contemporary form of the theory which properly retains the formers spirit. It is my contention that just such a theoretical advancement is available, once we understand that the Aristotelian dunamis which lies at the heart of that ancient form of essentialism is but the obscure, imprecise ancestor of the subsequently developed, specified and complex contemporary

7 This lack of evidence has arguably led to the formulation of the ,,Homeostatic Property Cluster view of natural kinds - see Boyd (1999), Wilson (1999), and Wilson, Barker, and Brigandt (2007). 8 See also Griffiths (2002), Hull (1992), and Sober (1980). 9 Cf. Wilson (1999: 190), and Okasha (2002: 197). 10 For a general overview, see Whitman & Agrawal (2009) and Schlichting & Smith (2002). 11 See Fusco & Minelli (2010) and West-Eberhard (2003).

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concept of dispositionality. The evolution of this central concept, I suggest, capably courts the viable evolution of ?.

Dispositional properties are inherently causal properties ? they are responsible for the coming about of particular states of affairs (,,manifestation states) upon the occurrence of some other state of affairs (,,stimulus conditions).12 These properties function as ontological "switches" of sorts, causally mediating the influence of certain activating conditions to produce particular states of affairs. The property of ,,negative charge, for instance, is widely understood dispositionally ? when its bearers meet with a like-charged particle (its stimulus condition), they repel with a particular momentum (its manifestation state). Of course, this (and examples of its ilk from "fundamental" ontologies) are the exception, for while its possible for dispositions to be realised by a single material element, they are more often than not realised by an entire system, or complex network of interacting elements: if upon receiving the appropriate conditions that network initiates a sustained step-wise progressive interplay among its nodes which leads to their production of a particular end state, that complex of elements realises a dispositional property.13

Thus dispositions are functionally defined with respect to their specific stimulus/manifestation pairs: whatever performs the function of causally mediating the occurrence of a specific manifestation state upon the occurrence of specific stimulus conditions is an instance of the disposition defined by that particular pair.14 Importantly, when we designate a particular structure as an instance of a functionally defined property, we are operating at a certain level of abstraction ? one that eschews the more specific details of the causal pathway by which that function is performed and focuses on the general end states between which that pathway runs. When a dispositional property is realised by a particular system then, the pathway from ,,stimulus to ,,manifestation often "reaches over" a wide, multi-stage causal gap ? thus, when such a gap is reliably and repeatedly bridged (upon the appropriate conditions being realised), we are afforded evidence of the existence of these properties.15

In abstracting to these end states, we not only abstract away from the particulars of that pathway ? that is, the various links comprising the causal chain between those states ? but also the various particular ways in which that pathway might be traversed. Accordingly, because there are many distinct instances of a particular type of stimulus condition which might lead to distinct instances of a particular type of manifestation state, the two states which define a disposition are determinables, not determinates. When a dispositional property is realised by a system then, that system is capable of producing a wide, gradientlike range of quantitatively distinct manifestation states, each representing a particular instance of its manifestation type, according to its particular stimulus input. Dispositional properties are therefore functional in a second sense, in that they establish a functional relation between a set of input values ? that is, particular determinate instances of certain determinable stimulus conditions - and a set of output

12 Dispositions are often contrasted with ,,categorical properties ? those whose nature must be imbued with causality from higher-order laws of nature, or else some flotilla of possible worlds. For discussions of the distinction, see Ellis (2010), Oderberg (2009), and Cross (2005). 13 Even the seemingly simple philosophers paragon of dispositionality ? ,,fragility ? is realised (in most cases) by a complex physical microstructure, and ,,breaking is in fact a complex, multi-stage process featuring the aligning of various micro-events that represent decreasing degrees of structural integrity. 14 That dispositional properties are responsible for establishing this type of causal connection between two states is the basis for their ubiquitous assignment as truthmakers for subjunctive conditionals (especially counterfactuals). However, spelling out precisely what the truthmaking role is, and showing that dispositional properties play it with respect to those conditionals turns out to be exceptionally tricky. As it happens, I wont be making use of that concept here, as Ive no need for it. For a good discussion of the related issues, see Austin (2015b) and Eagle (2009). 15 Ive said "reliably and repeatedly" purposely here, as dispositional properties do not necessitate their manifestations ? a fact ensured by the possibility of so-called ,,masks, properties or processes which interrupt the causal activity of dispositional properties. See Eagle (2009), Schrenk (2010), and Mumford & Anjum (2011) for good discussions of the issue.

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values ? that is, particular determinate instances of certain determinable manifestation states.16 In other words, more specifically, dispositional properties establish a causal link of functional co-variance of state-values between two variables.17

Because dispositions reliably and repeatedly produce particular end states, they are often understood as teleological, goal-directed properties ? they are causally "directed toward", and are thus for those ends. When a system is goal-directed toward a particular end, it exhibits the phenomenon of persistence18: it maintains the production of its end state "as a result of changes occurring in the system that compensate for any disturbances taking place (provided these are not too great) either within or [external] to the system, disturbances which, were there no compensating changes elsewhere, would prevent the realisation of the [end state]".19 The causal process which dispositional properties initiate and mediate is characterised by this sort of "course correcting" towards a particular end state, which they do in a systematic and non-accidental fashion ? that is to say, reliably and repeatedly, over a wide-range of changes/perturbations.20

In the framework of ?, organisms are ontologically sorted into natural kinds in virtue of sharing sets of causal properties which both generate and subsequently shape their morphological development it is my contention that these "powerful" properties are dispositional properties. The relevant question then is: is ?s ontology, armed with this contemporary gloss, consistent with our current understanding of the biological realm? I propose that it is ? for my claim is that the advent of evolutionary developmental biology has afforded us a unique view of that realm, one whose requisite ontology is dispositional, and whose foundational principles just are those of ?: for evo-devo is a framework in which morphological variation is derived from invariant, functional causal mechanisms which serve as highly conserved "deep homologies", underwriting a vast array of organismal diversity. In order to make the argument for the viable evolution of ? by way of evo-devo, I turn now to the specifics.

Recall that, if ? is to be plausible, there must be a set of causal capacities ? on my gloss, a set of dispositional properties ? jointly responsible for an organisms particularised morphological development. The question is: in any particular organism, are there discrete properties which function as generative mechanisms with respect to particular phenotypic traits? According to evo-devo, there most certainly are21: for one of the guiding principles of that framework is the modularity of development, according to which "...developmental systems are decomposable into components that operate according to their own intrinsically determined principles".22 These separable and distinct ,,developmental modules are identified with highly internally integrated genetic regulatory networks which "interpret" particular intra- and intercellular signalling into downstream (spatial and temporal) regulatory control via the production of transcription factors, resulting in patterns of expression which specify the particularised developmental pathways of discrete morphological structures.23 In other words, these modules are each responsible for the specified development of a particular morphological structure in a developing organism, and

16 In the dispositions literature, this fact is often referred to as dispositions being "multi-track". See Martin (2008), Manley and Wasserman (2008), Jacobs (2011), and Vetter (2013) for fuller discussions. 17 This is a species of the relation that Lewis (2000: 190) called ,,causal influence which forms the conceptual bedrock of Woodwards (2003; 2010) influential theory of causation. 18 Of course, not all dispositions are strongly goal-directed in this sense, but the ones which will concern us here ? namely, those that populate the biological realm - certainly are. 19 Nagel (1977: 272). 20 See Walsh (2012), and Mayr (1992). 21 Indeed, as many have now argued, ,,modularity may very well be a necessary requisite for the process of evolution: we may need variability to occur within discrete elements which doesn't affect other elements if organisms are going to survive mutations and be subsequently subject to selection pressures. See Lewontin (1978) and Altenberg (1995). 22 M?ller (2008: 10). 23 See Winther (2001), Bolker (2000), and Von Dassow & Munro (1999).

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accordingly, are individuated with respect to the structure whose development they are causally responsible for, evidence for which is gathered either by ectopic expression experiments24, or else by the principled decomposition of genotype-phenotype mappings.25 Indeed, although discovering the niceties of the regulatory architecture which comprises these developmental modules is an interesting and thus far fruitful research programme (especially with respect to its prowess in establishing molecular-based phylogenetic lineages), what is most important is their generative specificity with respect to particular morphological structures. This is underscored by the fact that these modules role in the production of such structures is characterised by a highly robust, degenerative process ? one underwritten by their constitutive genetic regulatory networks ability to maintain integrity by means of its non-isomorphic elements becoming isofunctional26 ? , and is one which, over time, and in successive generations, may become autonomised, gaining a kind of independence from their (original) underlying genetic mechanisms.27 Accordingly, causal explanations of the development of the structures associated with these modules eventually operate at high ,,causality horizons ? that is, at explanatory levels "above" the workings of its molecular constituents.28

Recall that if ? is correct, the ontological ground floor of organisms consists in collections of properties which are (jointly) causally responsible for their specified morphological development ? that is, in collections of dispositional properties, each "directed toward" the development of a particular morphological structure. Even on the general reading just given, I think its easy to see that ? is consistent with evo-devo: the fundamental ontological postulates of the latter ?,,developmental modules ? can be conceptualised as instances of the ontological cornerstones of the former. In other words, from the Aristotelian point of view, developmental modules are dispositional properties.29 The developmental modules of evo-devo are causally responsible for the specified production of their associated morphological structures in developing organisms in virtue of their serving as a functional bridge between intra- and inter-cellular signalling and specific downstream genetic expression patterns which initiate particularised developmental pathways resulting in the formation of those structures. These modules therefore function as ontological "switches", causally mediating the influence of certain activating conditions to produce particular states of affairs: given the appropriate stimulus conditions, developmental modules reliably and repeatedly produce particular end states.

And it is with respect to these end states (read: morphological structures) that these modules are functionally individuated: for what is important, theoretically, to the definition of a particular module is not the particularities of the genetic regulatory networks (or hierarchical sets of such networks30) which undergird its activity, but rather the role it plays in morphological development. Thus, these "higher order" modules are defined after the fashion of dispositional properties, at a certain level of abstraction ? away from the various complexities of the aforementioned particularities ? and are therefore able to be conceptually (and in some cases, as mentioned above, physiologically) "disassociated" from any specific underlying mechanism and constitutive processes. Not only are these modules functionally individuated

24 This technique was especially prominent in Halder et al. (1995); For a general contemporary review in a particular case, see Ashery-Padan & Gruss (2001). And for an analysis of the expression patterns of modules in particular, see Raff & Sly (2000). 25 See Wagner & Altenberg (1996). 26 See Edelman & Gally (2001), Von Dassow et al. (2000), and Whitacre & Bender (2010). 27 This is discussed with particular examples in M?ller & Newman (1999), and M?ller (2003). 28 See Salazar-Ciudad & Jernvall (2013) for the concept of ,,causality horizons in explanations of developmental morphology. 29 Though this general application has been made ? in Wagner (2000) and Eble (2005)? , it has only been very briefly stated, and not explored in any depth. 30 Its plausible that there are at least four distinct "levels" of morphological organisation ? see Rasskin-Gutman (2003).

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with respect to their end states, but they are plausibly "directed toward" those states ? for the degenerative robustness of their underlying networks is an instance of the dispositional, teleological phenomenon of persistence.31

Having satisfied the basic conceptual criteria, let us call these developmental modules, in line with the Aristotelian metaphysic, phenomodulatory dispositions. Of course, according to ?, its not enough that there exist collections of causal properties which are responsible for the morphological development of the organisms which possess them ? these collections must also be (a) shared among certain sets of organisms, representing a kind of ontological stability, or invariance, and yet somehow function to (b) ontologically ground the "accidental" morphological variation among those sets. The question is: are there shared sets of phenomodulatory dispositions among groups of organisms which function to ontologically underwrite a wide range of their morphological variation? According to evo-devo, there most certainly are: for one of the fundamental posits of that framework is the existence of conserved developmental resources whose inherent plasticity is the causal ground of phenotypic variation. Indeed, in a notable shift from the neo-Darwinian perspective, evo-devo favours a ,,structuralist approach32, wherein the diversity of organismal development is understood to be underwritten by a drastically less diverse set of developmental modules which themselves constrain and specify the variability of their associated morphological structures according to their own "generative rules".33

We now know that the morphological structure produced by a single developmental module, being underwritten by a particular genetic regulatory network, is capable of a wide variety of intra- and inter-cellular environmentally induced phenotypic variation: alterations in ,,positional information consisting mainly of heterochronical and heteropical changes in hormonal and endocrinal signalling ? results in qualitative alterations of the phenotypic character of that structure; this is the phenomenon of phenotypic plasticity, attested to by the reality (read: quantifiability) of reaction norms.34 This is possible because, as evidenced by the evo-devo paragon of HOX regulatory networks, these modules are situated in the "bottleneck" of the process of development, between a host of upstream signalling pathways and a certain set of downstream ,,target genes, the latter of which are responsible for producing the "building blocks" of particular morphological structures.35 Because these regulatory networks are thus positioned, they function as causal mediators, interpreting cascades of upstream "inputs" into downstream "outputs" via their production of transcription factors which enact (spatial and temporal) regulatory control at the cis-regulatory sites of downstream target genes which directly specify cell fate.36 Thus, alterations in their input values are causally correlated with corresponding alterations in their output values ? that is, with alterations in the qualitative character of their corresponding morphological structure.37

This fact brings with it two important points, the first being that a common set of modules possessed by the members of a single species is able to ground a wealth of their morphological variation for one and the same developmental module can be responsible for a wide variety of phenotypic variation in a particular structure as a result of (broadly construed) environmental influences: as described above, alterations in upstream signalling are interpreted by these modules into downstream regulatory control

31 Thus, in the context of dynamical systems theory, the morphological structures associated with these modules are often characterised as ,,attractor-states which shape the "valleys" of an organisms epigenetic landscape, resulting in many distinct developmental pathways leading to the same end-state. See Jaeger & Monk (2014), and Striedter (1998). 32 See Amundson (2005) for an excellent in-depth discussion of the ,,structuralist paradigm and its relation to that of the Modern Synthesis. 33 Cf. M?ller (2008). 34 See West-Eberhard (2003), and Pigliucci (2001). 35 For more on the concept of the ,,developmental hourglass, see Galis & Metz (2001) and Kalinka et al. (2010) 36 See Mann & Carroll (2002), Gurdon & Bourillot (2001), and Tabata (2001). 37 See Schlichting & Pigliucci (2002), and Aubin-Horth & Renn (2009).

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over a (shared) set of "building block" target genes, resulting in morphological changes in the development of that structure. Thus on this small, intra-species scale, the morphological variation of particular traits can be accounted for by the causal activity of a shared set of developmentally plastic modules. More important perhaps is the second, rather surprising point that even inter-species morphological variation of particular traits is often grounded in the causal activities of a shared set of developmental modules; this is the so-called phenomenon of deep homology.38 Here, as a result of distinct species possessing distinct sets of downstream target genes ? and hence, distinct sets of regulatory regions ? a single, shared developmental module, producing a particular set of transcription factors, is capable of controlling the specified formation of two seemingly distinct morphological structures. In other words, on evo-devos ,,structuralist framework, even the more extreme qualitative variation of a particular morphological structure present in two otherwise distinct groups of organisms is often causally underwritten by a shared developmental module.

Importantly, these modules which function as "units of stability" (Eble 2005: 223) underlying the morphological variation among seemingly intractably qualitatively diverse structures in distinct populations are themselves primarily responsible for that variation. On account of their aforementioned roles as causal mediators between upstream signalling pathways and downstream target genes, these shared developmental modules are not only responsible for the development of a particular morphological structure in distinct sets of organisms39, but also for variations on that structure: their generative competence in defining a traits ,,morphospace ? the representation of its possible structural permutations - is an intrinsic affair.40 In other words, it is their inherent plasticity that grounds these modules ability to function as the causal basis for the morphological variation on their associated structures, as the specification of their reaction norms - representing the functional relation between upstream signalling and downstream targets - is a role which is "immanent to the system".41

On the evo-devo framework then, there exist shared, discrete developmental elements within and among populations of species which are intrinsically causally responsible for the specified development of a certain generalised morphological structure, and which causally control the production of the various particularised forms of that structure. As I hope by now is clear, the operative ontology of that framework centres on phenomodulatory dispositions, and is theoretically consistent with the metaphysic of ?. As discussed above, phenomodulatory dispositions are functionally individuated, causally active elements "directed toward" the development of a particular morphological structure. Importantly however, dispositional properties are functional in a second sense, for the causal role they perform is one of mediating and specifying the causal co-variance of determinate state-values between two determinable variables. Thus, the developmental role of morphological modules in functionally mediating between upstream positional signals and the production of a particular morphological structure is a dispositional role ? they function to "interpret" specific, determinate collections of a generalised class of stimulus factors into specific, determinate forms of a generalised manifestation state. Due to this inherent plasticity of dispositional properties, one and the same disposition is able to causally underwrite a wide (though restricted) range of end state variations, and hence serve as the shared, "hidden" foundation of a diversity of qualitative attributes among seemingly fundamentally diverse sets of objects. As we have seen,

38 See Shubin, Tabin, and Carroll (2009), and Wagner (2007). 39 Wagner (2014: 92-93) argues that ,,character-identity determination (a) cannot be specified by positional information, given that they are variable in and among instances, and that it likewise (b) cannot be specified by downstream target genes, given not only their similar variability, but also their regulatory dependence upon upstream modules. 40 Cf. Newman and M?ller (2006), and Newman et al. (2006). For the concept of ,,morphospace generally, see Rasskin-Gutman (2005), and McGhee (2006). 41 M?ller (2008: 19).

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